Update assignment1.py

Author: Arun Kirubarajan

assignment1.py

@@ -57,7 +57,7 @@ def substrings(seq):
 
  Example:
  >>> substrings("abc")
- "a", "ab", "abc", "b", "bc", "c"
+ {"", "a", "ab", "abc", "b", "bc", "c"}
  """
  pass